This invention relates to polypropylene, particularly to polypropylene modified to produce biaxially oriented film for the packaging market. Biaxial orientation is well known in the industry and is used to enhance both the optical and physical properties of the polymer film. Contemporary commercial technology for specialty film involves composite structures that can contain as many as 5 to 7 different coextruded polymeric components. Coextrusion with polymers other than polypropylene is used to improve or impart special properties to the polypropylene such as impermeability or sealability.
To maintain compatibility and clarity of the composite, polypropylene must be used which is known to have a low haze property. Historically, high clarity polypropylene is obtained by modifying the polymer microstructure; the addition of a suitable nucleating agent (i.e. sodium benzoate) to the polymer during pelletizing is known to alter crystalline structure (spherulite growth) and enhance clarity. This improved clarity, however, can be at the expense of good processability and non-uniform properties in the film. For example, optical or mechanical defects such as tearing of the film during drawing can be caused by inhomogeneities (i.e. large particle size distributions) related to the presence of nucleating agents or other inorganic additives.
My invention provides for the production of a film grade polypropylene that is nucleated, easy to process, has exceptional multi-directional physical properties, and exhibits a low measure of haze. Such a combination of properties has been extremely difficult to achieve in the past.
Prior to the present invention, it has been known that bis(alkylbenzylidene) sorbitols could be used as clarifying agents, i.e. to reduce haze, in polyolefins. See the sales sheet for "Millad 3905" attached to the Prior Art Statement filed herewith; also see abstracts of the Japanese Patent application No. 81/186,663, Nov. 19, 1981; application No. 81/166,675, Oct. 19, 1981, and U.S. patent application No. 363,824, Mar. 31, 1982, now abandoned.
It is also known in the art that so-called "controlled rheology" (CR) in polypropylene can be attained by the closely regulated addition of certain peroxides such as 2,5-dimethyl-2,5 di(t-butylperoxy) hexane. The peroxide typically is added in the extruder during pelletization (after the polymerization process is complete). The peroxide will tend to encourage a more or less uniform breakdown of the molecular structure of the polypropylene under the physical and thermal stress of the extruder, resulting in a product having a relatively narrow molecular weight distribution; the reduction in average molecular weight and the improved narrowness of the molecular weight distribution are typically accompanied by an increase in melt flow and a change in both rheology and physical properties compared to those of a reactor product have a similar melt flow. This "cracking" technique is well known in melt spinning fibers and more recently has become accepted in high-speed injection molding; however, film made from low melt flow peroxide-cracked material is "flexible" and has a high percent haze, two undesirable properties for polypropylene films.
It has also been suggested in the prior art that so-called linear low density polyethylene may be used to make films. Linear low density polyethylene (LLDPE) is polyethylene containing a small amount (typically 0.5%) mono olefin co-monomer such as 1-hexene, 1-butene, or 1-octene and generally having a relatively narrow molecular weight distribution and short chain branching. See, for example, U.S. Pat. No. 4,362,835. It is indicated in German Pat. No. 1,902,573 that the incorporation of polyethylene (homopolymer) may have an effect on the crystallization rates of polypropylene.
The reader may be interested also in U.S. Pat. Nos. 4,359,544; 4,282,076 and 4,296,022 for general background in the area of nucleation and controlled rheology.